Sera from patients with halothane hepatitis contain antibodies directed against liver microsomal proteins (100 kDa, 80 kDa, 63 kDa, 59 kDa, 58 kDa, 57 kDa, and 54 kDa) covalently modified by the trifluoroacetyl halide metabolite of halothane. These altered proteins (neoantigens) are believed to cause an immune-mediated hepatotoxicity. In this project, the 80 kDa protein was purified from liver microsomes of halothane treated and untreated rats by ionic exchange chromatography. An antibody was raised against the protein in rabbits and was used to determine the subcellular localization and organ distribution of the protein. Quantitative immunoblotting studies revealed that the highest levels of the protein were found in the microsomal fraction of the liver, followed by approximately 50% lower amounts in the nuclear, mitochondrial, and plasma membrane fractions. Very low levels of the protein were detected in the cytosol fraction of the liver. The protein was detected in all tissues studied with the highest levels found in the liver, fat, and testes, while moderate levels were measured in all other tissues tested except for the heart and skeletal muscle which had low levels of the protein. When the amino acid sequences of several internal peptides produced by either tryptic or mild acid hydrolysis of the protein were compared to sequences of proteins in several data bases, it was discovered that the 80 kDa protein showed greater than 95% sequence homology with two cDNA clones. One of the clones encoded for ERp 72, an endoplasmic reticulum protein with unknown function. The other clone corresponded to deoxycytidine kinase, the rate-limiting enzyme in the activation of many important anticancer and retroviral drugs, including the anti-AIDS drug AZT.